The present invention relates generally to the field of circuit interrupting devices. More particularly, the invention relates to a technique for reducing the tendency of arcs created during interruption of a current carrying path to delay entry into an arc dissipation structure, such as a splitter plate stack.
A variety of devices are known and have been developed for interrupting current between a source and a load. Circuit breakers are one type of device designed to trip upon occurrence of heating or over-current conditions. Other circuit interrupters trip either automatically or by implementation of a tripping algorithm, such as to limit current to desired levels, limit power through the device in the event of phase loss or a ground fault condition, and so forth. In general, such devices include one or more moveable contacts which separate from mating contacts to interrupt a current carrying path. The devices may be single phase or include multiple phase sections for interrupting current through parallel current paths, such as in three phase applications.
It is desirable in circuit interrupter to limit the total let-through energy during interruption. The let-through energy is determined by a number of factors, and may be reduced by increasing the speed of interruption of the current through the device. A wide range of techniques have been employed for improving interruption times to limit the let-through energy. In a number of these techniques, arcs which develop between the contacts during interruption are caused to migrate towards dissipating structures, such as conductive plates arranged with air gaps between each plate. The voltage investment in the arc may be caused to rise very quickly to cause rapid interruption of the current. Where splitter plates or similar structures are provided for dissipating or conducting the arc, entry of the arc into these structures may be important in extinguishing the arc and thereby limiting the let-through energy during the interruption.
An arc created during interruption of current in a circuit interrupter may delay extinction for a variety of reasons, including due to a phenomena referred to in the art as xe2x80x9carc retrogression.xe2x80x9d As the arc migrates towards a splitter plate stack, the voltage across the plate stack builds to a point at which a new arc is developed outside the plates. This second arc provides a parallel path for fault current. Because the arc outside the plates presents less resistance to current flow than the arc inside the plates, the fault current rapidly switches to the new arc and the old arc inside the plates extinguishes. This action has the appearance of an arc moving backwards, hence the term xe2x80x9cretrogressionxe2x80x9d (also referred to as xe2x80x9cback commutationxe2x80x9d). Once the effect occurs, the new arc is pulled into the plates by the magnetic interaction of the arc current with the material in the plates, and the cycle repeats. Such retrogression presents difficulties in that it supports less back EMF on average than an arc that enters into a plate stack without successive arcs. The resulting lower voltage investment, in turn, leads to a longer time required to force the current to a zero level, and a greater Joule integral or let-through energy.
There is a need, at present, for an improved technique for reducing arc retrogression in circuit interrupters. There is a particular need for a technique which is effective at reducing let-through energy, while providing a straight forward and simple construction.
The present invention provides a novel approach to arc retrogression avoidance or reduction designed to respond to these needs. The technique may be employed in a wide variety of devices, including circuit breakers, circuit interrupters, contactors, and so forth. The technique may also be applied in a variety of device configurations. For example, arc retrogression in single contact structures may be addressed, as well as in structures in which a conductive spanner or contact bridge is displaced to create a pair of arcs on either side of the device (i.e., line and load sides). When used in conjunction with fast-acting circuit interrupting techniques, the arc retrogression reduction scheme permits extremely fast circuit interruption, enhancing the performance of the devices and reducing the let-through energy during interruption events.
In accordance with aspects of the present technique, contacts in a circuit interrupter are separated to create an arc during an interruption event. The arc is driven towards a splitter plate stack or a similar dissipating structure. An arc dielectric enhancing medium within the circuit interrupter is released to reduce the retrogression of the arc upon contact with and entry into the splitter plate stack. The arc dielectric enhancing medium may be provided in a current carrying path parallel to the arc, such that the medium is released upon generation of the arc during the interruption event. The arc dielectric enhancing medium may include a material released by surface ablation of a source element which is heated during the interruption event. A hydrocarbon gas serves as the dielectric enhancing medium in the embodiment described herein, and is released by surface ablation of a resistance-transitioning element. Transition of the resistance level of the element protects the element from damage during the interruption event, permitting it to undergo a number of interruption events while remaining affective at reducing arc retrogression.